The present invention is directed to a process for upgrading light paraffins such as ethane, propane, and butanes. Interest in upgrading these light paraffins has been growing due to recent and anticipated changes in refinery processing schemes which resulted and will result in a greater supply of such light paraffins. These changes include: the higher severity operation of the reforming process in order to maintain a high octane rating in the absence of or reduction of the lead content in gasoline; the lowering of reid vapor pressure (RVP) specifications; the increased use of oxygenates such as methyl tertiary butyl ether (MTBE) and ethanol resulting in the removal of butanes from the gasoline pool; the increased demand for jet fuel necessitating increased gas oil hydrocracking resulting in more light gas production; and the increase in operating temperatures in fluidized catalytic crackers resulting in more light gas production. Thus, there is great incentive to investigate means for converting these materials into more valuable liquids such as transportation fuels or chemical feedstocks.
The upgrading or conversion of light paraffinic gases and synthesis gas has previously been carried out in the presence of gallium-based or gallium-containing catalysts.
U.S. Pat. No. 4,543,347 (Heyward et al.) discloses a catalyst composition suitable for converting synthesis gas to hydrocarbons which is a mixture of zinc oxide and an oxide of at least one metal selected from gallium and iridium, an oxide of at least one additional metal collected from the elements of Group IB, II through V, VIB and VIII including the lanthanides and actinides and a porous crystalline tectometallic silicate.
U.S. Pat. No. 4,490,569 (Chu et al.) discloses a process for converting propane to aromatics over a zinc-gallium zeolite. This zeolite optionally may also contain palladium. More specifically, the catalyst composition used in the instant patent consists essentially of an aluminosilicate having gallium and zinc deposited thereon or an aluminosilicate in which cations have been exchanged with gallium and zinc ions wherein the aluminosilicate is selected from the group known as ZSM-5 type zeolites.
U.S. Pat. No. 4,585,641 (Barri et al.) discloses crystalline gallosilicates which may be impregnated, ion exchanged, admixed, supported or bound for catalyzing a reaction such as alkylation, dealkylation, dehydrocyclodimerization, transalkylation, isomerization, dehydrogenation, hydrogenation, cracking, hydrocracking, cyclization, polymerization, conversion of carbon monoxide and hydrogen mixtures through hydrocarbons and dehydration reaction. The metal compounds which may be used for ion exchange or impregnation may be compounds of any one of the groups of metals belonging to Groups IB, IIB, IIIA, IVA, VA, VIB, VIIB and VIII according to the Periodic Table. Specifically, preferred compounds include copper, silver, zinc, aluminum, gallium, indium, vanadium, lead, antimony, bismuth, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, radium, thorium and the rare earth metals. Patentees describe their gallosilicate as "Gallo Theta-1" in contradistinction to an MFI-type gallosilicate which has a substantially different x-ray diffraction pattern.
U.S. Pat. No. 4,350,835 (Chester et al.) relates to a catalytic process for converting gaseous feedstocks containing ethane to liquid aromatics by contacting the feed in the absence of air or oxygen under conversion conditions with a crystalline zeolite catalyst having incorporated therein a minor amount of gallium thereby converting the ethane to aromatics. The gallium is present in the catalyst as gallium oxide or as gallium ions if cations in the aluminosilicate have been exchanged with gallium ions. The patent further discloses that the original alkaline metal of the zeolite, when it has been synthesized in the alkali metal form, may be converted to the hydrogen form or be replaced by ion exchange with other suitable metal cations of Groups I through VIII of the Periodic Table, including nickel, copper, zinc, palladium, calcium or rare earth metals.
European Patent application No. 0 107 876 discloses a process for producing an aromatic hydrocarbon mixture from a feedstock containing more than 50 wt. % C.sub.2 through C.sub.4 paraffins. Specifically, the process is carried out in the presence of crystalline gallium-silicate having a SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of 25 to 250 and a Y.sub.2 O.sub.3 /Ga.sub.2 O.sub.3 molar ratio lower than 1 where Y can be aluminum, iron, cobalt, or chromium. The disclosure also teaches a two-step silicate treatment comprising a coke deposition and a coke burn-off with an oxygen-containing gas.
European Patent Application No. 0 107 875 similarly discloses a process for producing an aromatic hydrocarbon mixture from a feedstock comprising more than 50 wt. % of C.sub.2 through C.sub.4 paraffins. This process is carried out in the presence of a crystalline gallium-silicate, having a SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of 25 to 100 and a Y.sub.2 O.sub.3 /Ga.sub.2 O.sub.3 molar ratio lower than 1 where Y can be aluminum, iron, cobalt, or chromium.
Light paraffinic gases have also been upgraded to liquid aromatics in the presence of crystalline aluminosilicate zeolite catalysts having incorporated therein a minor amount of a metal selected from Groups VIII, IIB, and IB of the Periodic Table. For instance, U.S. Pat. No. 4,120,910 (Chu) discloses copper-zinc-HZSM-5, platinum-HZSM-5, copper-HZSM-5, and zinc-HZSM-5 catalysts suitable for converting a gaseous paraffinic hydrocarbon feed to aromatic compounds.
It has now been discovered that C.sub.2 through C.sub.5 light paraffins can most effectively be upgraded to aromatics by the catalytic process of the present invention minimizing methane and ethane production while simultaneously minimizing the production of undesirable heavy aromatics such as naphthalenes.